the alu must be dirty because it doesn't rust in the same way steel does. It quickly (seconds) builds up a layer of Al2O3 on the outside, which is commonly increased in depth via anodyzing. The oxidation is normal aluminium in color unless pigment has been mixed in with the anodyzing (as is common). You should be able to clean it with citric acid (Orange cleaners) pretty easily.
If its rust on a model F plate be careful as they are comprised of a few layers.